Water shortage is now a growing global concern. The supply of water resources does not keep pace with an increasing demand and the contamination of water leads to the deficiency of available water. At present, roughly one third of the world's population lives in areas that lack water. It is estimated that about two third of the world's population will suffer from water shortage by 2020 (R. F. Service, Science 313, 1088 (2006)). Under such circumstances, considerable research efforts have been made to produce clean water by desalinating or purifying seawater, river water, and contaminated water.
Established commercial techniques for producing clean water are, for example, based on distillation and reverse osmosis using semi-permeable membranes through which particles other than water do not pass. However, according to the distillation, boiling water consumes energy as much as 10-16 kWh/m3. According to the reverse osmosis, much energy is needed (4-6 kWh/m3) because a pressure should be applied that is greater than the osmotic pressure of a solution to be removed.
Forward osmosis (FO) has recently received attention as future technology for water desalination and purification due to its higher energy efficiency than distillation and reverse osmosis. Forward osmosis is performed in two steps. In the first step, a water species requiring desalination or purification and a solution having a higher osmotic pressure than the water species are arranged to face each other through a semi-permeable membrane. This solution is referred to as a “draw solution” and a constituent solute of the draw solution is referred to as a “draw solute.” With this arrangement, water migrates to the draw solution by osmosis even without additional energy supply. In the subsequent second step, the solute is technically removed from the draw solution that has drawn the water. According to the forward osmosis, clean water can be obtained even by very low energy consumption.
The choice of a suitable draw solute having an optimum osmotic pressure and the removal of the draw solute from clean water after desalination or purification are key considerations in forward osmosis. Ammonium bicarbonate (NH4HCO3) is currently the most widely studied draw solute in forward osmosis. Ammonium bicarbonate is dissociated into ammonium (NH4+) and bicarbonate (HCO3−) ions. A high concentration solution of ammonium bicarbonate can draw water from seawater, river water, and contaminated water through osmosis due to its high osmotic pressure. After water has been drawn using ammonium bicarbonate, heat at 60° C. is supplied to remove the solute. The heat vaporizes the solute ammonium bicarbonate into ammonia (NH3) and carbon dioxide (CO2). As a result, the water solubility of the gases is extremely lowered, enabling the production of clean water.
However, several limitations exist in forward osmosis using ammonium bicarbonate. First, there is a risk that the basic ammonium bicarbonate solution may damage the physical properties of semi-permeable membranes typically composed of polymeric materials. Another limitation is that high energy is still needed to reach a temperature (60° C.) for removal of the solute. Besides maintaining the temperature at 60° C., removal of the draw solute requires the use of a complex column distillation process, which is also a disadvantage of forward osmosis. Another problem of forward osmosis is that the reuse of the draw solute involves a complicated process.
In this connection, Korean Patent Publication No. 2011-0091153 discloses a draw solution for forward osmosis. Specifically, the draw solution is an aqueous solution containing a draw solute in the form of a salt consisting of an anion and a cation, typically an ammonium ion. The draw solute is vaporized and separated from the aqueous solution. However, the inherent problems of forward osmosis using ammonium bicarbonate remain unsolved and the consumption of high energy is needed to reach a high temperature of 40° C. to 90° C., which is much higher than room temperature.
Despite many efforts to solve the disadvantages of the prior art, any of the draw solutes proposed to date is still unsatisfactory. Thus, there is a need to develop a new concept of draw solute and a forward osmosis system using the draw solution.